help a first-time Airtable user log in who had been initially invited 3 years ago.

We're trying to help an Airtable user on our team to log in for the first time.
He had been invited originally back in Feb. 2020. I'm not sure that he had signed up to get a password—probably not but maybe.
When he tries to access his account, he appears to be taken through loops that are dead-ends. 
When he has clicked on Continue, he got a blank page. 
So, we deleted his account and created a new one (with the same email address), and sent him a link.
When he clicked it, he got an "Email already in use" message. Please help. Thank you!